They sold advertising space on the make believe notion that x number of eyeballs saw every ad. It was always bogus.
No, they didn't.
At no point did anyone in publishing/advertising believe that putting a 3-inch single column ad in the Lifestyle & Arts section F of a newspaper with a circulation of 347,000 meant that the ad was seen by 347,000 different people. The fact that you personally may have never looked at ads (which, if you're being honest, is almost certainly not true unless you were born after 1990ish) doesn't matter. Everyone in publishing/advertising understood that they weren't getting 347,000 "pageviews" - they were making sure that their ad had a high chance of being seen/noticed by the percentage of readers who see/notice ads. And up until 2005-2010, that was still a large percentage.
Back before ~2010, and especially once you go back to 1960-2000, Americans deliberately, voluntarily, looked at advertisements in newspapers and magazines because that was how you found out what stores were out there, what the stores sold, what current prices were. It was common for people to dellberately flip through the inserts for stores like Sears, Montgomery Ward, JCPenney, Dillards, Kmart, WalMart, REI, Circuit City, Best Buy, as well as all the inserts and wrappers for local grocery stores, car dealerships, real estate listings, garage sale listings, movie theatre showtimes, on and on.
Ads were a normal, accepted part of the socioeconomic biome. It sounds wild now, but we used to live in harmony with them. When you didn't care about them you could easily ignore them. When you cared about them - and every adult at some point did care about them, to buy a car, a refrigerator, rent an apartment, find the lowest price on brisket, get the Domino's pizza coupons for this week's big game - you could easily find them and were glad they existed.
The modern seething revulsion toward advertising is a creation of the Internet, where ads escaped ecological equilibrium and evolved into invasive species which consume every possible space alongside the content and, when that space was not enough, the content itself became an ad - "I built this AI agent that lowered my car insurance by $130 month and you can too! Link in bio."
Nor were all ads the same. Which section, where on the page, sizing, layout, full color, section wrappers, extra inserts, etc. all had different costs. When ads weren't invasive - when they just sat there on a page rather than popup, flash, wiggle, trigger effects when your mouse happened to hover over, autoplay sound, autoplay video, drive-by download malware, etc.etc - you saw the ads that might matter to you, organically. If you didn't need to buy a car and didn't find cars interesting, you never opened the Automotive section of the paper, so none of those ads were ever seen by you. But those who were into cars or were in the market for a car, did look at that section and did see those ads. Same for people who were or were not interested in art galleries, the symphony, cultural festivals, cinema.
It's quite ironic that the aggressive targetmaxxing of Google and the other major Internet advertising companies - by insisting on chasing the reader down and holding our eyeballs open and showing us a constant stream of ads based on a phrase we used one time in a discussion board post somewhere - have actually decreased the variety and relevance of the ads we see. Because instead of it being an organic ecosystem where we see the ads we need when we naturally would want to see them, we now desperately wish to avoid seeing any ads, anywhere, at any time, under any circumstances. Even when they might actually be relevant - indeed, the targeted intrusiveness is often what we resent most, nowadays.